In manufacture of circular disc-shaped or square plate-shaped product comprising a material, such as Sendust alloy, cobalt alloy, high class high speed steel or an alloy mainly composed of Laves compound and/or intermetallic compound, which is difficult to be rolled or forged into a plate, it has been a general practice to prepare a round or square billet by casting, then slice it to obtain a circular disc-shaped or square plate-shaped product and, if necessary, grind its sliced surfaces. For example, high density magnetic recording has recently been progressed and Sendust alloy (Fe-Al-Si alloy) sputtering has come into use in manufacture of corresponding better magnetic heads. Since it is very difficult to plastically work this alloy, a target material as a mother material of sputtering has been cut into a plate directly from a billet prepared by casting. Also, in an alloy mainly composed of rare-earth-Fe type Laves compound and used in a recording medium of optomagnetic recording system, a target is cut directly from a cast billet since it is difficult to be plastically worked as in the case of Sendust alloy.
When a material which causes significant segregation in casting is used, it has been tried to cut a billet prepared from a powdered material by using a technique of hot press, hot isotropic press, hydraulic forging press or the like. Moreover, as a method other than slicing, it has been undertaken, since olden times, to hot-press and sinter a thin powder layer into a plate.
In the method of slicing a billet into a number of plate-shaped pieses, the slicing cost is high regardless of the method of preparing the billet and it is further raised due to poor production yield attributable to cutting margins. When the material has especially poor machinability, it is sometimes unable to cut by a conventional tool and it sometimes cracks even by a carbide tool, thereby singnificantly reducing the production yield. When it is sliced by using a special technique such as electrospark machining, electron beam cutting or lasar cutting, it requires a long working time and further reduces its productivity.
In addition, when the abovementioned Sendust alloy or rare-earth/Fe type alloy is cast into a billet, it frequently segregates in the way of solidification and may result in local deviation of composition from its predetermined value or internal gross porosities and cracks which make the billet unusable and widely reduce the production yield. When the casting technique is used, there is fair chance for producing rough crystal grains above one millimeter in the billet. In this case, the billet is so brittle that it is very difficult to cut it into plate-shaped targets and grind them, since cleavage crack occurs easily through the grain.
On the other hand, in the method of preparing a billet or plate-shaped product by hot-pressing a powdered material, there are upper limits or temperature and pressure such as 1,000.degree. C. and 1,000 Kg/cm.sup.2 according to industrial practice which is attributable to restriction of hot strength of a pressing die. Therefore, it is difficult to prepare a poreless sintered body of 100% density by hot-pressing for some kind of powdered alloy. When the resultant plate-shaped product including some remaining pores is used as a target material, it may be subjected to such a trouble in that thermal stress is concentrated around the pores to cause cracks therefrom or that a gas as an impurity is discharged from the pores to affect the sputting effect. Moreover, when the plate-shaped product is prepared one by one by hot-pressing, the productivity is further reduced.
In order to remove these troubles, a technique has been developed as disclosed in the Japanese patent opening gazette No. 1-306507. According to this technique, as shown in FIG. 1, powder layers 1 or a material to be formed into plates and partition plates 2 are alternately piled up and contained in a cylindrical capsule 3 made of workable metal and the capsule 3 is tightly closed, heated and then pressed within a die. The product is then cooled and metallic part attributable to the partition plates 2 and capsule 3 is removed. The materials of the capsule 3 and the partition plates 2 preferably have a low affinity to the powder to be treated and easily separable therefrom.
In this method, however, it is difficult to obtain a uniform thickness of the powder layer 1 and, therefore, the resultant plate-shaped product having a diameter of 150 mm, for example, may have an uneven thickness such as 7 mm plus/minus 2 mm and also include pores in its metallic structure.